<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>

    </script>
    <style>
        .weiyuansu{
            width: 100px;
            height:100px;
            border:1px solid red;
            font-size:16px;
        }
        .weiyuansu::before{/* 伪元素  */
            width:20px;
            height:20px;
            border-radius: 50%;
            background-color: blue;
            content: '';
            display: inline-block;
        }
        .weilei:hover{/* 伪类 */
            cursor: pointer;
            width:20px;
            height:20px;
            background-color: pink;
            display: inline-block;
        }
        .smallerFontSize{
            font-size: 20px;
        }
    </style>
</head>
<body>
    <div class="smallerFontSize">
        更小的字体
    </div>
    <div class="weiyuansu">
        伪元素是两个::    
    </div>
    <div class="weilei">
        伪类是一个:
    </div>
    <div>
        移动端布局:
            rem: 相对于根元素的字体大小
            em:相对于父元素的字体大小
    </div>
</body>
</html>
